1. Field of the Invention
This invention relates to the field of methods for preparing polymers. More particularly, it relates to the field of preparation of polymers that are useful as drag reducing agents in hydrocarbon transportation pipelines.
2. Background Art
The use of poly-alpha-olefins and copolymers thereof to reduce the effect of friction (“drag”) experienced by a liquid hydrocarbon flowing through a hydrocarbon transportation pipeline is well-known in the art. Reduction of the drag decreases the amount of energy needed to accomplish such flow, and therefore also decreases the costs associated with pumping. These materials, called drag reducing agents, can take various forms, including certain polymers in oil soluble suspensions, emulsions, pellets, gels, microfine powders, and particulate slurries. However, particulate slurries, comprising ground polymers, are often the least expensive form. The ultimate goal is a drag reducing agent which rapidly dissolves in the flowing hydrocarbon and which has a polymer content sufficient to ensure that the desired level of drag reduction is achieved.
The polymers which are most frequently used in preparing drag reducing agents are poly-alpha-olefins of carbon chain lengths ranging from 2 to about 40. Frequently these polymers are prepared using Ziegler-Natta catalysts and frequently also co-catalysts such as alkyl aluminum compounds. These polymerization reactions tend to be very efficient, producing relatively high yield when carried out in bulk. However, they also tend to be highly exothermic. The exotherm itself creates problems which reduce the usefulness of the product if the exotherm is not effectively alleviated. These problems include, in particular, a substantial reduction in the polymer molecular weight. Such molecular weight loss can result from even relatively minor deviations from a preselected optimal temperature, and can substantially reduce the efficacy of the polymer in a drag reducing agent formulation.
Those skilled in the art have attempted to reduce or otherwise control this exotherm in order to improve the quality of the polymers being produced. Some attempts to accomplish this have included, for example, carrying out the polymerization reaction in specially designed reaction bottles, wherein reactor layers ostensibly provide a level of protection from oxygen and water as potential sources of contamination. The bottles are stored in a cold room during the polymerization in order to reduce the exotherm by providing better heat removal from the reaction bottles.
Another method of addressing the exotherm problem for drag reducing agent polymers has been to use a screw conveyor as the reactor. This method is, however, ill-suited to producing larger quantities of polymer per batch, since in such cases a large screw conveyor must be used to achieve a commercially acceptable polymerization time. Unfortunately, the larger the conveyor, the less effective the heat transfer, and the less effective the heat transfer, the poorer the temperature control and, hence, the quality of the final drag reducing agent polymer.
Accordingly, what is needed in the art is a method of polymerization that preferably enables relatively tight control and/or overall reduction of the exotherm resulting from polymerization, such that high quality drag reducing agent polymers, as well as other polymers which would otherwise encounter similar undesired exotherm problems, can be produced in bulk.